In 1965, Theodore R. Freeman and his family were barred from joining a Fairfax County swimming pool club. The Freemans were black — the first black family in a previously all-white neighborhood — and swim club officials said their presence would drive others away.

Paul E. Sullivan, a white federal employee who lived nearby with his wife and seven kids, protested the club’s decision as racist. So officials at the Little Hunting Park pool association stripped the Sullivans of their pool membership, too. And Paul Sullivan found a lawyer.

The legal fight raged for nearly five years and cleaved the neighborhood, a subdivision just south of Alexandria’s city limits.

The Sullivans received countless threats by late-night phone call; their mailbox was blown up more than once. Theodore Freeman got a job abroad and his family moved, and still Mr. Sullivan persisted, bent on defending principle.

His stubbornness was rewarded in 1969, when the U.S. Supreme Court ruled that the swimming pool association had broken the law in turning away the Freemans.

Paul Eulalius Sullivan, the lead plaintiff in the landmark civil rights case Sullivan v. Little Hunting Park Inc., which helped prompt the desegregation of neighborhood swimming pools and other privately operated clubs across the country, died March 14 at his home in Fairfax County of complications from a stroke. He was 87.

His interest in the civil rights movement may have been rooted in his experience growing up Irish and poor in Boston.

Born May 26, 1923, he was the son of immigrants from County Galway who tried to protect their eight children from stereotype and discrimination by forbidding them from speaking or learning the Irish language.

Both of Mr. Sullivan’s parents died by the time he was 10. Raised largely by his older siblings, he joined the Navy during World War II and served in San Diego.

He used the G.I. Bill to study linguistics at Georgetown University, from which he graduated in 1951. When he was learning Spanish, he liked to attend dances held by the Latin American community. At one of them, he met his future wife, Guatemala native Flora Lopez-Enriquez, who had recently arrived to work for the World Health Organization.

They married in 1949. Besides his wife, survivors include five daughters, Isa Robins of Richmond, Maire Snodderly of Oklahoma City and Chela Sullivan, Maria D. “Lola” Sullivan and Monica Edwards, all of Fairfax County; 12 grandchildren; and three great-granchildren.

Three children preceded Mr. Sullivan in death: Sean C. Sullivan in 1958; William F. Sullivan in 1972; and Brigid Sullivan Healy in 1992.

The Sullivans started their family in the early 1950s and bought a home in Fairfax County’s Bucknell Manor development. Along with the house, they received a membership share to the new Little Hunting Park pool.

A decade later, the family converted that house into a rental and moved a few blocks away to another home, which also came with a pool membership. When Mr. Sullivan leased his house, he included use of a pool membership with the $129 monthly rent.

That worked smoothly for the first several renters. But those families were white.

When the Freemans — Theodore, a U.S. Department of Agriculture economist with a doctorate from the University of Wisconsin, and his wife and two children — moved into the neighborhood, the pool association notified Mr. Sullivan that they were not welcome to swim.

Mr. Sullivan tried to gather support from neighbors to overturn that decision; in response, the pool association terminated the Sullivans’ membership. Mr. Sullivan told The Washington Post several years later that he reacted with “absolute and utter confusion. Here is a Christian community that talked about decency, fair play and sportsmanship,” he said, “and they were treating people like this.”

He and Freeman filed suit in Fairfax Circuit Court seeking $30,000 in damages and reinstatement of both pool memberships. They lost when the judge ruled that the pool was a private club and thus exempt from civil rights laws — it could exclude whomever it wanted.

As appeals made their way through the courts over the next several years, neighbors took vehement sides.

“If they let in the coloreds, I’d close the pool down,” an anonymous longtime resident told The Post in 1969. “I’d sell it or close it.”

“I think it is very un-American not to let Negroes in,” another resident said. “We are a country built on equality and that means blacks and whites as well.”

Mr. Sullivan asked his priest at Alexandria’s St. Louis Catholic Church to address the divisive issue from the pulpit. The priest refused, according to Mr. Sullivan’s family, saying it would not be appropriate for the church to get involved.

Disgusted, Mr. Sullivan transferred to a different Alexandria parish — St. Joseph’s, then a predominantly black church. He worshiped there and sang in the gospel choir for the rest of his life.

The Supreme Court heard arguments in Mr. Sullivan’s case in fall 1969. The Sullivans were present when the opposing side’s lawyer put forth his argument. He said the pool was a private club that had chosen to deny the Freemans’ membership to maintain economic solvency: The family’s presence might cause a flood of members to resign.

Turning away the Freemans had nothing to do with the fact that they were black, the lawyer assured the justices, including Thurgood Marshall, who had been questioning him closely. “It’s not race,” the lawyer said.

“Could I say it’s just color?” Marshall said, as laughter rose in the gallery.

Mr. Sullivan was represented by Allison Brown, a renowned civil rights lawyer working pro bono on the case. Brown argued that the pool engaged in illegal housing discrimination: The pool membership was part of the lease and couldn’t be withdrawn simply on the basis of race.

The court agreed with Brown’s argument in a 5-3 decision written by William O. Douglas, who ruled that the pool was no “private club” because it wasn’t selective — except on the basis of race. “What we have here,” wrote Douglas, “is a device functionally comparable to a racially restrictive covenant.”

Several months later, the Sullivans and Freemans received a total of $5,150 in damages. A Fairfax judge ordered the pool to stop discriminating and to reinstate the Sullivans’ membership.

The case was seen as profoundly significant in the Washington area, where a wave of middle-class black families was moving into predominantly white suburbs, and where neighborhood pools were a popular tool to lure homebuyers to new subdivisions.

In 1970, Montgomery County cited Mr. Sullivan’s case when it ordered 46 community swimming pools to sign declarations promising racial desegregation. Soon afterward, a black doctor and his family were refused membership at a Silver Spring pool. They sued in a case that ultimately reached the Supreme Court, which unanimously upheld the Sullivan decision in 1973.

Mr. Sullivan and his wife continued to live in their Bucknell Manor home, and he retired in 1980 after a 30-year career as a Defense Department intelligence analyst.

In recent decades, he had become enthralled with his heritage and traveled often to Ireland. Despite his parents’ prohibitions, he took Irish immersion courses and became fluent enough to teach the language at Hagerstown Community College.